I’ve done it this way with spectacular results. The only reason why you speak without the mic is to ensure that your timings are correct in terms of syncing with the video.
As far as getting the video in a format that will be supported by Cubase, it depends (I suppose) on the version of Cubase you are running. You’ll find good video format converters for free on the web. I forgot what program I use - not at home for the next week or so - but it allows me to convert to the “standard” QuickTime format, which I know Cubase supports.